FIG. 1 is a typical circuit block diagram of an existing liquid crystal display (LCD), which includes a source driver IC, a gate driver IC, a timing controller (TCON) and a power supply circuit. The power circuit provides operating voltages to the source driver IC, the gate driver IC and the TCON. Usually, the power supply voltage is low, and the operating voltages different from the power supply voltage are required to be applied to the source driver IC, the gate driver IC and the TCON, so it is required to convert, i.e., step up or step down, the applied power supply voltage by the power supply circuit. Generally, a booster circuit is used as the power supply circuit of the LCD so as to step up the power supply voltage, and FIG. 2 is a topological diagram of the booster circuit.
Referring to FIG. 2, the booster circuit includes a DC power supply U0 and a DC booster circuit. The DC booster circuit includes an inductor L, a switch transistor M, a diode D, a capacitor C and a load Rf. The inductor L is connected in series between one input end of the DC power supply U0 and an anode of the diode D. One end of the switch transistor M is connected to the anode of the diode D and the inductor L, and the other end thereof is connected to another input end of the DC power supply U0. The anode of the diode D is connected to the inductor L and the switch transistor M, and a cathode thereof is connected to the capacitor C. The capacitor C is connected in parallel to both ends of the load Rf. Usually, the switch transistor M is a MOSFET.
In the DC booster circuit, a step-up effect is achieved by controlling an on state and an off state of the switch transistor M. When the switch transistor M is turned on, the current passes through the inductor L and the switch transistor M, the energy is accumulated in the inductor L, the current is supplied to the load Rf with a capacitor C, and the diode D is used to cut off a loop circuit through which the capacitor C is discharged via the switch transistor M. When the switch transistor M is turned off, the second diode D is tuned on, and the capacitor C is charged under the effect of counter electromotive forces of the DC power supply U0 and the inductor L. Hence, an output voltage Ui of the DC booster circuit is greater than a DC input voltage U0 of the DC booster circuit, and the DC booster circuit can be used to step up the voltage. Moreover, the output voltage of the DC booster circuit is associated with an ON time, i.e., an operating frequency, of the switch transistor M.
For the power supply circuit of the LCD, there mainly exist the power consumption for turning on the circuit and the power consumption for turning on or off the switch transistor. The former may be reduced via suitable elements, e.g., a diode having relatively low DC resistance, and the latter is generated by the gate capacitor discharge when the MOSFET is turned on or off. When the load is heavy, the power consumption for turning on the circuit constitutes a main portion of the total power consumption, and when the load is light, the power consumption for turning on the circuit decreases, and a ratio of the power consumption for turning on or off the switch transistor to the total power consumption increases. Hence, it is required to reduce the operating frequency of the switch transistor, thereby to maintain the power supply circuit at high operating efficiency.
However, currently the DC booster circuit can merely operate at one operating frequency. An output voltage of the DC booster circuit is a constant value, and the current passing through the load is inversely proportional to the load. Hence, when the load in the power supply circuit changes, the operating efficiency of the power supply circuit will change too, which thus results in a decrease in the operating efficiency of the power supply circuit. Moreover, when the load is changed to be light, the current in the power supply circuit increases, which thus results in the conversion of excessive electric energy to heat and an increase in an overheating risk of the power supply circuit. As a result, a normal operation of the power supply circuit, and even that of the LCD, will be adversely affected.